Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 280531

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
131 AM EDT Fri Jul 28 2017

A strong cold front will cross the region from the west Friday
through Friday night. Cooler and drier air will arrive across the
western Carolinas and northeast Georgia through the weekend and
linger through the early part of next week. More typical summertime
conditions will return about the middle of next week.


130 AM EDT Update...No major changes were made to the fcst grids.
PoP was adj down across the nrn zones a bit based on latest hi-res
consensus and radar trends. Temps and td/s look right in track and
still expect best fg/stcu chances across the mtn valleys and
fthills...possibly into the Upstate thru the morning.

As of 1015 PM: Trimmed PoPs back a bit over the next few hrs, with
HRRR and NAMNest showing just some very isolated activity firing on
remnant outflows left behind by the current storms. I continue to
advertise an increase from north to south beginning after midnight
as the convergence zone and associated showers encroach from the
north. Temp trends were largely on track so far; the guidance blend
chosen by the previous shift accounted for the cirrus well. I did
make some minor adjustments to values later tonight as the newest
hourly guidance is overall a bit warmer. This is reflected in new
min temps.

The 18z NAM bucked the trend of late and showed less of a
stabilizing effect Friday due to the morning cirrus and possibly
light stratiform precip. 00z NAM continues this trend. Our
confidence has increased that at least the NC Piedmont, where
model consensus has been highest for significant instability,
will be at risk for severe wx. At least the GFS sweeps a cold
pool from upstream storms into our forecast area such that it
depicts virtually no sbCAPE across most zones by 18z. While
this is plausible, it`s by no means a given, and it does create
a considerable amount of uncertainty in tomorrow`s forecast. The
uptick in instability seen in the most recent NAMs is decreasing the
uncertainty for severe wx in our east, in that it lends confidence
to other solutions still showing high SBCAPE values there. However
it increases the uncertainty in how far west the advertised SPC
Slight Risk should be extended when Day 2 becomes Day 1 tonight.

At any rate, the potential for abundant buoyancy is a bit
disconcerting considering the anomalously strong mid/upper level
flow, with attendant deep layer forcing and low level & deep
layer shear of 25-35 kts, as one would typically expect a very
active severe weather day with such ingredients in late July. The
combination of upper support, along with lee side/pre-frontal
trough and perhaps outflow boundary(s) associated with the morning
convection should support scattered to numerous showers and storms
across the area by afternoon. The potential for a few significant
severe storms still cannot be ruled out. Tomorrow`s highs should
be a couple of degrees (if not more) below climo under expected
widespread mid/high clouds.


As of 225 PM EDT Thursday: The best upper-level forcing rounding the
slowly deepening eastern trough will likely cross our piedmont
Friday evening before swinging rapidly east to the coast overnight.
An attendant sharp cold front will also cross the region through the
nighttime hours. PoPs will thus remain fairly high during the
evening hours before drier air starts arriving from the northwest
overnight post-fropa. However, the best severe convective threat
should have passed east of the area by evening - unless any strong
redevelopment occurs along the immediate frontal zone. Showers may
linger along the northern mountains in moist low level upslope flow
well into Saturday morning.

A closed, upper-level low pressure system will then migrate from the
central Appalachians to the Virginia Tidewater region Saturday
through Saturday night. The surface cold front will depart southeast
of the region and drier air will work in. The main exception is that
a trailing vorticity lobe rounding the upper low will swing down
across the western Carolinas Saturday night, reinforcing moisture in
the 850 to 700 mb layer. This will lead to some return cloudiness,
but probably no associated showers.

Deep layer northerly flow will continue Sunday through Sunday night
on the back side of the deep eastern coastal trough. Periods of mid
level moisture could permit slightly higher cloud cover at times,
but with no attendant showers expected. Anticipate maximum
temperatures about 1 to 2 categories below climo by Sunday


As of 225 PM EDT Thursday: Reinforcing shortwave energy arriving
from the upper Midwest will keep a mean trough axis positioned over
the eastern U.S. Monday through Wednesday. Mainly dry surface high
pressure will sprawl over to the north of the region through this
same period. The airmass will slowly modify, with isolated
convective coverage returning to the southeast mountains late Monday
and then more typical diurnal mountain ridge top coverage Tuesday
through Thursday. Some modest height recovery is expected on
Thursday with max temperatures finally returning to near seasonal


At KCLT and elsewhere: Morning -shra looks to be limited to KHKY and
possibly KAVL and KCLT thru daybreak. Not expecting much lowering of
cigs or vsby thru the early period outside oh KAVL and KGSP due to
upper clouds relatively high sfc tdd/s. KAVL and the Upstate will
see lowering vsby and some stcu this morning due to saturated llvls
and a stagnant airmass. The rest of the TAF period will focus on the
eastward advancement and tstm activity associated with a passing
cold front. With upper Ci thickening and holding on thru much of the
period...think the best chance for restrictions will be with the
storms themselves. Thus...have MVFR vsby included in
afternoon/evening Prob30s all sites. A lowering of cigs is expected
behind the front late in the period to MVFR possibly IFR.

Outlook:Drier air will filter in from the north behind a passing
cold front this weekend. Chances for morning fog and stratus
will also be possible, Saturday morning, mainly in the mountain
valleys. A typical summer-time pattern is expected to return by
the middle of next week.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     Low   26%     High 100%     Low   56%
KAVL       High 100%     High  95%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     Med   60%     High 100%     Med   62%
KAND       High 100%     Low   38%     High 100%     Med   69%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:




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